The invention relates generally to a spoked wheel that includes a hub, a wheel rim, and a plurality of spokes that transfer torque from the hub to the rim. More specifically, the invention relates to a spoked wheel that uses flexible spokes.
Performance bicycle wheels are a compromise between weight, and static and dynamic stability. While reducing weight, structural strength must be maintained.
The spokes of a bicycle wheel and their lacing pattern determine the static and dynamic stability of the wheel. This is most important in rear wheels, because the spokes couple the driving torque from the hub to the wheel rim. Torque transfer must occur with maximum efficiency to maximize the energy exerted by a cyclist.
A typical spoked wheel has a first set of spokes under tension on one side of the wheel, coupling the rim to a corresponding hub flange and a second set of spokes under tension on the opposite side of the wheel, coupling the rim with a corresponding hub flange. The two hub flanges are set at an axial distance from each other. When the wheel is viewed in section, the hub, spokes, and rim approximate a triangle.
The spokes on the two sides of the wheel have a camber angle with respect to the median plane of the wheel. The camber angles relate to a wheel's dish. The angles cause the spoke tensioning to give rise to force components in a direction parallel to the axis of the wheel. Balancing the force. components keeps the rim centered in the median plane.
A rear wheel hub carries at one end a sprocket cassette which is part of the bicycle transmission (drive train) and requires axial space. The spokes set on the drive train side have camber angles that are smaller than the camber angles of the spokes on the opposite side. This requires the smaller camber angle spokes be tensioned more than the opposite side spokes that have greater camber angles in order to maintain the rim position in the median plane of the wheel. Different spoke camber angles may also appear in front wheels, where the hub may be occupied by a brake disk. However, most symmetrically dished front wheels carry less weight and do not have to deal with large torsional loads.
FIG. 1 shows a partial section view of a prior art rear bicycle wheel 101. The wheel 101 comprises a hub 103, a sprocket cassette 105 coupled to the hub 103, a rim 107 and tire 109. The hub 103 is coupled to the rim 107 via A (drive train) spokes and B (non-drive side) spokes. The wheel's median plane M is orthogonal to the hub axis X midpoint. The A spokes located at the drive train hub flange 111 have a camber angle α with respect to the wheel's median plane M. The B spokes located at the non-drive side hub flange 113 have a camber angle β with respect to the wheel's median plane M. α is less than β. Each spoke is tensioned with a given tensile force. Corresponding tensile component vectors TA and TB are applied to opposite sides of the wheel 101. The horizontal vector components TAO and TBO of TA and TB are in a direction parallel to the axis X.
For wheels having an equal number of spokes on each side of the wheel, the horizontal vector components TAO and TBO must be balanced with one another. These forces maintain the rim 107 in the median plane M. However, the tensile force TA must be greater than the tensile force TB due to its smaller camber angle. The ratio between the tensile force TA and TB must be approximately equal to and opposite to the ratio of the sines of the camber angles α and β. This template applies to each pair of opposing spokes and as a sum with reference to the total tensile forces of the spokes on one side and the opposite side.
Conventional spokes have at one end threads for engaging a nipple to couple the spoke to a rim, and at the other end an elbow and a head for coupling with a bore of a hub flange. Spokes are made of different types of materials and may be butted, with reduced thickness of the spokes at the center section. The nipple is used to adjust spoke tension. The nipple is usually located at the rim end of the spoke, but may be located at a hub flange. Spokes are usually circular in cross section, but may be flat or oval cross-sectioned to improve rotation aerodynamics.
Most bicycle wheels on single rider bicycles have 28, 32 or 36 spokes, while wheels on tandem bicycles have 40 or 48 spokes. Wheels with fewer spokes have an aerodynamic advantage as the aerodynamic drag from the spokes is reduced. However, fewer spokes results in a larger section of the rim being unsupported, thereby requiring stronger rims.
Conventional spoke lacing patterns that transfer torque from the hub to the rim for driven wheels, or wheels with drum or disc brakes, typically require a tangential lacing pattern. The spokes leave the hub at an angle close to 90° (tangential) or at various angles, and usually cross other spokes to the rim.
Tangentially laced wheels transfer torque because one half of the spokes, called leading spokes, point in the direction of rotation, while the other half, called trailing spokes, point in the opposite direction. The leading and trailing spokes counteract each other when no torque is applied. When forward torque is applied during acceleration, the trailing spokes experience a higher tension while the leading spokes relax. The opposite occurs when braking, with leading spokes experiencing greater tension and trailing spokes relaxing. Leading and trailing spokes allow for the transfer of force in either direction, minimizing tension changes, and due to symmetry, allows the wheel to stay true regardless of the torque applied.
Wheels which are not required to transfer significant amounts of torque from the hub to the rim may use radial lacing. In radial lacing, the spokes leave a hub flange at zero degrees without crossing another spoke. Radial lacing cannot adequately transfer torque because torque on the hub would induce a stress in the hub flange bore, spoke elbows and nipples, and rim, increasing the likelihood of failure in any one of them. Radial lacing increases the stress on the hub flange since spoke tension pulls straight at localized points. While radial lacing uses shorter spokes which minimize weight, it is offset by the need to use a stronger hub. However, radially-laced wheels are stiffer and more precise than other lacing patterns.
A mix of radial and tangential lacing may be used on rear wheels with tangential lacing on the drive train side and radial lacing on the opposite side. Most of the torque is transferred by the drive train side of the hub while the opposite side stabilizes the wheel. A wrong-way, half-radial lacing may be used, with radial lacing on the drive train side and tangential lacing on the opposite side. This accounts for wheel dish, the drive train side spokes have greater tension and should not be burdened with transmitting drive torque. This design requires the hub to transmit torque from the drive train side to the opposite side. Many other lacing patterns exist. However, most are for aesthetic reasons.
What is desired is a bicycle wheel that offers reduced weight while allowing for increased performance.